Flotation is a physical separation process for separating fine-grained solid mixtures, such as ores and tailings for example, in an aqueous slurry or suspension with the aid of air bubbles on the basis that the particles contained in the suspension possess a different surface wettability. Flotation is employed for conditioning natural resources found in the earth and in the processing of preferably mineral substances having a low to medium content of a usable component or a valuable resource, for example in the form of nonferrous metals, iron, rare earth metals and/or noble metals as well as non-metallic natural resources.
WO 2006/069995 A1 describes a pneumatic flotation cell having a housing comprising a flotation chamber, with at least one nozzle arrangement, referred to here as ejectors, additionally with at least one sparging device, referred to as aeration devices or aerators when air is used, as well as a collecting tank for a foam product formed in the course of the flotation process.
In pneumatic flotation, a suspension composed of water and fine-grained solid matter to which reagents have been added is generally injected into a flotation chamber by way of at least one nozzle arrangement. The desired effect to be achieved by the reagents is that in particular the valuable particles or valuable resource particles in the suspension that are preferably to be separated are rendered hydrophobic. In most cases xanthates are used as reagents, in particular in order to selectively hydrophobize sulfide ore particles. Simultaneously with the suspension, the at least one nozzle arrangement is fed with gas, in particular air, which comes into contact with the hydrophobic particles in the suspension. The hydrophobic particles adhere to gas bubbles that form, such that the gas bubble structures, also referred to as aeroflocks, float to the top and form the foam product on the surface of the suspension. The foam product is discharged into a collecting tank and typically also thickened.
The quality of the foam product or the degree of success of the flotation separation method is dependent inter alia on the collision probability between a hydrophobic particle and a gas bubble. The higher the collision probability, the greater is the number of hydrophobic particles that will adhere to a gas bubble, ascend to the surface and form the foam product together with the particles.
In this case a preferred diameter of the gas bubbles is less than approximately 5 mm and lies in particular in the range between 1 and 5 mm. Such small gas bubbles have a high specific surface area and are therefore able to bind and entrain considerably more valuable resource particles, in particular ore particles, per volume of gas used than larger gas bubbles.
Gas bubbles having a larger diameter generally rise faster than gas bubbles of smaller diameter. In the process the smaller gas bubbles are gathered up by larger gas bubbles and aggregate with the latter to form even larger gas bubbles. This results in a reduction in the available specific surface area of the gas bubbles in the suspension to which the valuable resource particles are able to bind.
In flotation cells embodied in the shape of a column, in which a diameter of the flotation chamber is less by a multiple than its height, the distance that a gas bubble has to travel in the suspension or the flotation chamber in order to reach the surface of the suspension is particularly great. Due to the particularly long distance traveled, particularly large gas bubbles are produced in the suspension. The specific yield of valuable resource particles from the suspension decreases as a result, and consequently the efficiency of the flotation cell is also reduced.
In implementations referred to as hybrid flotation cells, which represent a combination of a pneumatic flotation cell with a flotation cell embodied in the shape of a column, larger valuable resource particles having particle diameters in the range of 50 μm and greater in particular do not bind fully to the gas bubbles present and so can only be partially separated from the suspension. In contrast, fine fractions with particle diameters in the range of 20 μm and less are precipitated particularly well.
In order to ensure that gas bubbles having a diameter in the range of 1 to 5 mm are present continuously over the height of the flotation chamber in a flotation cell embodied in a column shape, it is necessary to reduce the diameters of the gas bubbles generated in the lower section of the flotation chamber or by means of a sparging device in the flotation chamber. In certain conventional flotation treatment solutions use is made of sparging devices having gas outlet orifices whose diameters range from 3 to 5 mm and which lead in column-shaped flotation cells to a gas bubble formation having gas bubbles that are much too large, in particular greater than 5 mm in diameter.
Any further reduction in the diameters of the gas outlet orifices of sparging devices is virtually impossible in practice. Consequently, gas outlet orifices having diameters of up to 1 mm on sparging devices easily become clogged when suspensions that are typically to be processed having a solid matter content in the range of 30 to 40% are used. Even during short downtimes of the flotation cell, particles from the suspension infiltrate the gas outlet orifices and block them. When the cell is restarted, the pressure of the gas that is to be introduced into the suspension is frequently insufficient to flush out such small gas outlet orifices of a sparging device so that they are free again.
It is all the more important for this reason to take measures already at the injection point in order to prevent the gas bubbles injected into the suspension from combining to form large gas bubbles.
U.S. Pat. No. 1,583,591 describes an arrangement for treating liquids with gases and for use in the flotation treatment of ores, wherein an atomizer, or rotary gas diffusion member, is used.
GB 1272047 describes an air sparging device for aerating effluent slurries, said device comprising a cylindrical chamber which has an inlet opening at one end thereof for feeding oxygen or air thereto, and in addition has a plurality of outlet openings, each outlet opening comprising a conduit extending radially from the wall of the chamber and having a transverse cross-sectional area less than the cross-sectional area of the chamber. The air sparging device may be used in a rotating mode of operation in order to improve the aerating action.
However, rotating parts in the suspension are subjected to increased wear and tear, in particular in the flotation treatment of suspensions having a high solid matter fraction, such as in the flotation of ores.